China is one of the countries which have the most serious problems of municipal refuse.
According to statistics, the yield of municipal domestic refuse in the whole country was 115 million tons in 1980s, and had been 143 million tons in 1990s, which was the second in the world, next only to United States.
At present, the total amount of stockpiled municipal refuse in past years is up to 7 billion tons, and the amount of yield increases at a rate of about 8.98% per year. The stockpiled refuse occupies a total land area of up to 500 million square meters, which is converted to about 750 thousand mu of arable land.
70% of 600 large and middle cities in the whole country are surrounded by refuse, and the urban development and the living environment of surrounding residents are severely affected.
In our country, current refuse treatment substantially employs conventional methods such as landfill, composting, incineration, etc. However, the conventional refuse treatment techniques described above have well known defects.
I. Refuse Composting Treatment
The refuse composting treatment is a biochemical process which decomposes organic components in refuse using microorganisms. In the process of the biological chemical reaction, by means of the interaction of organics, oxygen and bacteria, carbon dioxide, water, and heat are emitted, and at the meanwhile, humus is generated.
The refuse composting treatment has the following three disadvantages.
1. Wastes, such as blocks, metal, glass, plastic, etc., cannot be decomposed by microorganisms, and these wastes must be sorted for further treatment;
2. The refuse composting has long period, large floor area, and poor sanitary conditions near refuse composting fields;
3. The compost has low efficiency, high cost, difficult selling, and poor economic benefit.
The refuse composting treatment is directed to organics capable of being decomposed by microorganisms, and thus it is a treatment technique only directed to organic components in refuse, not a final treatment technique for the entire refuse.
The refuse composting technique has the remarkable disadvantage that indecomposable organics and inorganics cannot be treated, and thus has low degrees of volume reduction, amount reduction, and harmlessness. Therefore, the problems of refuse cannot be thoroughly solved only by composting treatment. In some locations at present, domestic refuse is landfilled for nature fermentation, it is then excavated after several years, and sold as a fertilizer after indecomposable substances such plastic, etc., therein are screened out. Actually, this fertilizer is dangerous to plant fruit trees, vegetables, and crops for the reason that indecomposable organic components (paper, plastic, cloth, rubber, etc.) in refuse has a high content of heavy metals (Pb, Cd, Hg). If these substances are buried in the ground together with decomposable organics for a long term, along with the effect of rainwater, heavy metals will consequentially permeate final organic compost products, and foods produced using these composts will certainly have excessively high contents of heavy metals, which is harmful to the health of human.
II. Refuse Incineration Treatment
The refuse incineration is a process of delivering refuse to an incineration furnace for combustion. The thermal energy is released after the incineration of refuse, and flue gas and solid residue are generated at the meanwhile. The thermal energy needs to be recovered, the flue gas needs to be cleaned, and the residue needs to be digested. These are indispensable processes in the incineration treatment.
The refuse incineration treatment has the following disadvantages.
1. Toxic gases, such as dioxin, mercury vapor in batteries, etc., are released during refuse combustion. Flue gas generated must be cleaned, but the cleaning techniques are difficult and has high operation cost;
2. The process of refuse combustion generates toxic and harmful slag and dust, the amount of which is about 20%-30% of the original amount. The toxic and harmful slag and dust generated in incineration must also be treated.
3. It is a resource-wasting treatment method, as resources capable of being recovered and reutilized in refuse are destroyed
4. There are certain requirements for the low heat value of refuse in the incineration treatment, and not all types of refuse may be incinerated.
5. One-time investment for incineration equipment is high.
III. Refuse Landfill
The refuse landfill treatment has the following disadvantages.
The landfilled refuse does not subject to an innocent treatment, and a large amount of bacteria, viruses still remain; latent dangers such as marsh gas, pollution of heavy metals are hidden; and the effused liquid of refuse will pollute groundwater resources for a long time. Therefore, this method has extremely large potential dangers, and will bring out endless troubles to descendants. This method does not achieve reutilization treatment of refuse, but also occupies a large amount of land and leaves pollution sources to descendants. The most important is that the treatment capacity of each landfill field is limited and after filling, additional investment is still needed to construct a new landfill field, further occupying land resources. At present, refuse landfill is prohibited by official order in a number of developed countries.
Based on overall analysis, none of refuse treatment techniques used in our country at present is a method which can actually achieve the overall goals of refuse reutilization, innocuity, and amount reduction.